In recent years, an optical element made of glass is utilized in a wide range of applications such as a lens for a digital camera, an optical pickup lens for an optical disc, a camera lens for a cell phone and a coupling lens for optical communication. As such an optical element made of glass, a molded glass body manufactured by press-molding a glass material by use of a mold has come to be generally utilized.
As such a manufacturing method of molded glass bodies, proposed is a method in which molten glass droplets are dropped on an under mold and the dropped molten glass droplets are subjected to press-molding with the under mold and an upper mold facing to the under mold to prepare molded glass bodies (hereinafter, referred to as “a liquid droplet molding method”). This method has been noted because time necessary for one shot of molding can be made very short since it is possible to manufacture a molded glass body directly from a molten glass droplet without repeating heating and cooling of the molds.
However, in this liquid droplet molding method, a minute concave part is formed in the central neighborhood of the bottom surface of a molten glass droplet (the contact surface with an under mold) due to collision at the time of dripping a molten glass droplet on an under mold. Since air getting in this concave part has no escaping place to be kept sealed until a molten glass droplet is cooled and solidified, there was a problem of an air pool remaining on the bottom surface of a glass molded body manufactured.
To solve this problem, proposed is a method in which the surface of an under mold is made to have Rmax of 0.05-0.2 μm and secure a flow path of air having got in a concave part to prevent an air pool from remaining (refer to patent document 1). Further, proposed is an under mold which prevents an air pool as well as makes easy reproduction by forming a cover layer on the surface of an under layer having Ra of 0.005-0.05 μm (refer to patent document 2).